1. Technical Field
The invention disclosed broadly relates to semiconductor processing and more particularly relates to improvements in photoresist techniques for reactive ion etching in semiconductor processing applications.
2. Background Art
Present reactive ion etching techniques with a single layer resist system, suffer from two deficiencies, the first is the inability to rework photoresist layers after they have been developed, and the second is the problem of variability in etched metal line widths due to reflections from substructures. Reference to FIGS. 1-6 will illustrate the problem of the prior art in the variability of line widths due to unwanted reflections from substructures. A convention is adopted in FIGS. 1-6, to illustrate the relative orientation of the views. FIG. 1 includes a set of three orthogonal coordinates x, y and z. The view in FIG. 1 is the x-z plane. FIGS. 2-5 are in the y-z plane. FIG. 6 is in the x-y plane. FIG. 1 shows a starting composite of a substrate 20 upon which lies a layer 22 of silicon dioxide, a layer 24 of an aluminum copper alloy, a layer 26 of polycrystalline silicon, on top of which has been deposited a layer 28 of a positive photoresist. The silicon dioxide layer 22 has a region C which is thinner than the region A, so that an inclined step region B must make the transition to join the region A to the region C. It is in such regions as the step region B that unwanted diffraction patterns occur in the light used to expose the photoresist layer 28, causing unwanted variations in the resulting width of the exposed portion of the photoresist and consequently the resultant width of the reactively ion etched metal lines. This can be seen with reference to FIGS. 2 and 3. FIG. 2 shows the cross-sectional view in region A along the section line 1A--1A' of FIG. 1, where it is seen that a patterned light 30 is used to expose the photoresist layer 28 so that the portions 28' are exposed and will therefore be subsequently developed and removed whereas the portion 28" is not exposed and therefore will remain in place after development. In comparing FIG. 2 with FIG. 3, FIG. 3 illustrates the cross-sectional view of the composite of FIG. 1, in the region B for the inclined step. In the region B, the exposure light 30 passes through the photoresist layer 28 and is reflected off the polysilicon and aluminum copper layer 26 and, by virtue of the inclined surface of the polysilicon layer 26 in the region B, a diffraction pattern 32 occurs causing light to pass into the central region 28" of the photoresist layer 28, thereby creating an unwanted exposure of the region 28" along its edges. FIG. 4 shows the next step in the processing of the prior art photoresist layer 28, by using a developer such as potassium hydroxide to develop the photoresist layer 28 which can be for example a novolac-type photoresist. The photoresist region 28" in region A is shown to be wider after development than the photoresist layer 28" in region B. This, as previously mentioned, is due to the diffraction pattern of light 32 which occurs in the region B, which allows the developer solution to dissolve an additional portion of the photoresist 28", thereby making it narrower. FIG. 5 shows the next step in the prior art processing technique where a reactive ion etching takes place, making use of the photoresist pattern as a mask when reactively ion etching the polysilicon layer 26 and the aluminum copper layer 24. As can be seen from FIG. 5, the resultant line width for the polysilicon layer 26 and aluminum copper layer 24 in the region B is narrower than is the resultant line width in region A. This can be better seen by the top view shown in FIG. 6 where a wider line width results for the polysilicon layer in region A than in region B. This is an undesirable feature of the prior art's technique and is a result of the lack of control of the diffracted light 32 from substructures such as that occurring in region B. The prior art has attempted to cure this problem by adding absorptive dyes to the photoresist layer 28 to reduce the amount of light reflected from the polysilicon and aluminum copper layer 26 back up into the photoresist layer 28. The problem with this prior art solution is that it reduces the photo activity of the photoresist, since the light 30 coming in from the top must be of a greater intensity in order to adequately penetrate down to the bottom of the photoresist layer 28. The presence of the absorptive dye in the photoresist layer 28 creates a large gradient in the amount of exposure which occurs between the top and the bottom of the photoresist layer and this presents problems with regard to the resultant vertical contour of metal structures etched after the development of the photoresist.
Another problem with the prior art is the inability to rework the photoresist structures once they have been exposed and developed. This is seen in the illustration of the prior art in FIGS. 7-9. In FIG. 7, the photoresist layer 28 has been applied on top of the polysilicon layer 26 as previously described and the photoresist layer is exposed to the patterned light 30 as previously described. This results in the exposure of photoresist in the region 28' which will be subsequently removed by the development step and the non-exposure of the central region 28" which will not be removed after the development step. FIG. 8 shows the development step where using conventional developers such as potassium hydroxide or sodium metasilicate which react with and dissolve the photoresist layer at the portions 28' which were exposed to the light 30 but which do not dissolve the portion 28" which was not so exposed. The problem with the development step is that the developer solutions such as potassium hydroxide also significantly etch the exposed surface of the polysilicon layer 26. The etched surface 34 shown in FIGS. 8 and 9 remains as a permanent feature of the surface of the polysilicon layer 26 after the development step. It is necessary to carry out the development step so that the remaining photoresist structures 28" can be examined by means of an optical microscope to determine their alignment relative to other structures on the integrated circuit chip. This alignment measurement is very critical since misalignment of the metal structures which are to be etched will result in shorts, opens and other failure modes throughout the integrated circuit. If it is determined that there is a misalignment, then a rework cycle must take place where the photoresist structures 28" are removed from the surface of the polysilicon layer 26 and a new layer of photoresist must be applied and exposed with the patterned light. The problem can be seen in FIG. 9 in that the inadvertently etched portion 34 for the polysilicon layer 26 remains intact and presents a difficult surface to rework because of its confusing reflectance when it is to be examined in subsequent alignment measurement steps. Typically, an integrated circuit wafer that has gone through photoresist development and which has been determined to have misaligned photoresist structures, must be scrapped because of the latent image etched intothe polysilicon layer 26 by the developer.